Anthony Edwards played Dr. Mark Greene. He was on ER from seasons 1-8.
On a side note, the actress Mare Winningham was in 4 episodes of ER in season 5 (1998). She played a crazed doctor with an unhealthy obsession with Dr. Greene. Anyhow, in real life Mare and Anthony have been friends for 35 years. During that time, Mare was married 4 times and Anthony was in a 21 yr marriage that ended in 2015. At the end of 2021, Anthony and Mare eloped and in the twilight of their lives are blissfully married to each other.